An ion exchange membrane fuel cell, more specifically a proton exchange membrane (PEM) fuel cell, produces electricity through the chemical reaction of hydrogen and oxygen in the air. Within the fuel cell, electrodes, denoted as anode and cathode, surround a polymer electrolyte to form what is generally referred to as a membrane electrode assembly, or MEA. Oftentimes, the electrodes also function as the gas diffusion layer (“GDL”) of the fuel cell. A catalyst material stimulates hydrogen molecules to split into hydrogen atoms and then, at the membrane, the atoms each split into a proton and an electron. The electrons are utilized as electrical energy. The protons migrate through the electrolyte and combine with oxygen and electrons to form water.
A PEM fuel cell includes a membrane electrode assembly sandwiched between two graphite flow field plates. Conventionally, the membrane electrode assembly consists of random-oriented carbon fiber paper electrodes (anode and cathode) with a thin layer of a catalyst material, particularly platinum or a platinum group metal coated on isotropic carbon particles, such as lamp black, bonded to either side of a proton exchange membrane disposed between the electrodes. In operation, hydrogen flows through channels in one of the flow field plates to the anode, where the catalyst promotes its separation into hydrogen atoms and thereafter into protons that pass through the membrane and electrons that flow through an external load. Air flows through the channels in the other flow field plate to the cathode, where the oxygen in the air is separated into oxygen atoms, which joins with the protons through the proton exchange membrane and the electrons through the circuit, and combine to form water. Since the membrane is an insulator, the electrons travel through an external circuit in which the electricity is utilized, and join with protons at the cathode. An air stream on the cathode side is one mechanism by which the water formed by combination of the hydrogen and oxygen is removed. Combinations of such fuel cells are used in a fuel cell stack to provide the desired voltage.
The flow field plates have a continuous reactant flow channel with an inlet and an outlet. The inlet is connected to a source of fuel in the case of an anode flow field plate, or a source of oxidant in the case of a cathode flow field plate. When assembled in a fuel cell stack, each flow field plate functions as a current collector.
Electrodes, also sometimes referred to as gas diffusion layers, may be formed by providing a graphite sheet as described herein and providing the sheet with channels, which are preferably smooth-sided, and which pass between the parallel, opposed surfaces of the flexible graphite sheet and are separated by walls of compressed expandable graphite. It is the walls of the flexible graphite sheet that actually abut the ion exchange membrane, when the inventive flexible graphite sheet functions as an electrode in an electrochemical fuel cell.
The channels are formed in the flexible graphite sheet at a plurality of locations by mechanical impact. Thus, a pattern of channels is formed in the flexible graphite sheet. That pattern can be devised in order to control, optimize or maximize fluid flow through the channels, as desired. For instance, the pattern formed in the flexible graphite sheet can comprise selective placement of the channels, as described, or it can comprise variations in channel density or channel shape in order to, for instance, equalize fluid pressure along the surface of the electrode when in use, as well as for other purposes which would be apparent to the skilled artisan.
The aforementioned PEM fuel cells are being developed as an alternative energy source for portable, stationary, and industrial applications. Significant R&D efforts in the fuel cell area are being directed towards the science of fuel cell technology as well as in the areas of engineering and systems integration. A common need at the heart of all PEM systems is to increase the understanding of molecular level interactions within the system including gas flow to the membrane electrode assembly (“MEA”), diffusion, kinetics, thermodynamics of reactants and products of the electrochemical reaction, water management, heat transfer, and current collection.
Presently, diagnostic systems like fuel cell test stations are available which allow performance testing of stack-level component integration, combined with electronic measurements for performance evaluations, these systems are very costly, complex, and time consuming to operate. Additionally, individual component characterization and material evaluation is potentially possible through the use of classic electrochemical, and materials characterization methodologies such as X-ray diffraction, Potentiostatic/Galvanostatic measurements, impedance analysis, and microscopy.
As an example of industry shortcomings in the testing regime, Gurley porosity is commonly utilized to give an indication of the permeability of a fuel gas (e.g., hydrogen) through gas diffusion layer substrates. While Gurley porosity is useful for initial material screening purposes, direct correlation to operational performance is difficult. Also Gurley porosity does not include any specificity towards a correlation with the electrochemical reaction that takes place at the anode or cathode. Furthermore, localized differences in gas diffusion rates are difficult to detect.
There is a lack of availability, of intermediate testing paradigms that elucidate material and component integration, below the stack-level or even single cell level integration (ex-situ). Also there is need for testing methods to evaluate component performance functions under conditions that simulate real fuel cell operation. Furthermore, there is a need for a quick cost-effective testing paradigm for components.